1. Field of the Invention:
This invention relates to a distance measuring device and more particularly to an active type automatic distance measuring device having light projecting means.
2. Description of the Related Art:
The conventional automatic distance measuring device of a camera has been arranged to have a focus in the central part of a photographing picture plane, as shown in FIG. 13 of the accompanying drawings Therefore, in cases where a main object to be photographed is not in the central part of the picture plane, a distance measuring zone is first adjusted to the main object and, after that, the camera is adjusted to a desired picture (composition) before releasing a shutter as shown in FIG. 14. This action which is called pre-focusing has been necessary.
With the main object not located in the central part of the picture plane, if the shutter is released without pre-focusing, the main object would be out of focus to give a so-called "center-passing" picture as the focus is obtained only in the central part of the picture.
To solve this problem, there has been proposed an automatic distance measuring device which is arranged to measure distances not only for the central part but also for other parts of the picture and to perform a focusing action according to the distance data thus obtained. This is called a multi-point distance measuring action or a wide-field distance measuring action (see FIG. 15). For the multi-point distance measuring action of a so-called active type distance measuring device which has light projecting means and is arranged to project a high-frequency-modulated light onto an object and to measure a distance to the object through light reflected by the object, it is a conceivable method to provide the device with a plurality of light projecting means (or elements).
In this instance, however, if these light projecting means are arranged to serially light up one after another for measuring distances for corresponding distance measuring points, a long period of time is required for the distance measuring action. For a photographing operation, such arrangement causes a so-called time lag to become excessively long.
To solve this problem, it is also conceivable to arrange these light projecting means to be simultaneously lighted up to obtain measured distance information for the different distance measuring points. However, that arrangement presents the following problem:
The camera is operated by a battery disposed within the camera. The battery supplies energy for driving the light projecting means. The amount of the driving energy, therefore, increases with the number of light projecting means employed. A large number of light projecting means might cause the battery to become unable to supply the driving energy in a sufficient amount. In order to supply the sufficient amount of energy for simultaneously driving a large number of light projecting means, the camera must be provided with either an expensive or large battery unsuitable for the camera.
Further, in cases where the light projecting means are driven through a modulation process, the energy from the battery is intermittently supplied to the light projecting means. If infrared-ray emitting diodes (hereinafter referred to as iREDs) are employed as the light projecting means, for example, an iRED driving current intermittently flows. Therefore, a current supplied from the battery also intermittently flows. The terminal voltage of the battery, i.e., the power supply voltage, is caused to drop by the internal resistance of the battery when the current flows and comes back to the original voltage when no current flows. The power supply voltage thus varies in synchronism with the modulation phase of the iRED as shown in FIG. 16.
Meanwhile, a light receiving circuit which receives the reflection light coming from the object is arranged to detect a signal component by performing so-called synchronous detection or synchronous integration in synchronism with the modulated phase of the iRED, that is, in synchronism with the phase of the signal component.
Therefore, if the variation of the power supply voltage of the light receiving circuit causes the varying degree thereof which is synchronous with the iRED modulation phase, i.e., the phase of the signal component, to come to the signal system of the light receiving circuit, it results in an error in detecting the signal component. In other words, it causes a measured distance data error.
Generally, to minimize this error, the power supply part is provided with a capacitor of a large capacity; or a filter is used for suppressing the power supply voltage variation of the light receiving circuit; or the light receiving circuit is arranged in such a way as to prevent as much as possible the signal system from being affected by the power supply voltage variation.
However, in cases where a plurality of light projecting means are used to increase the required amount of driving current, the terminal voltage variation of the battery increases to cause the degree of the abovestated error to become no longer negligible.